Each target file has its own state file, to avoid concurrent access.
Make context separator(in email body) configurable
New feature: config option "init_scan_lookback"

Introduce new config option "init_scan_lookback" to control how many
blocks to look back for the first scan (when state file is absent).
New feature: config email subject

Improve configuration syntax so that email subject can either be
extracted from context (by regex) or specified as static text.
Tiny clean up
Update sample config file